I really hope I am wrong, but it sounds like something died on the PC. Like a video card or monitor are the 2 things that come to mind imediately. The description that you gave has the sounds of something that is having a problem with overheating and shutting it down for an hour let it cool off, and then you could turn it back on until whatever it was heated up again. Some pieces will eventually go out completly if they over heat too many times. There are a couple of other possibilites, but this seems like the most likely.

The above is only an educated guess.

When you turn your computer on do you hear anything like the fan comming on or that beep that you hear on alot of computer, several seconds after the power is turned on.

Let me know.